The preparation of aqueous, stable, i.e. non-sedimenting, dispersions of anionic, cationic or non-ionic polyurethanes which have been modified to render them hydrophilic has been known for a long time (see, for example, British Pat. No. 1,076,688, German Pat. No. 1,184,946, German Auslegeschrift No. 1,495,745, German Offenlegungsschriften Nos. 1,495,847; 2,035,732; 2,344,135 and 2,446,440 and U.S. Pat. Nos. 3,479,310; 3,756,992 and 3,905,929; "DIE ANGEWANDTE MAKROMOLEKULARE CHEMIE" 26 1972, pages 85 to 106; "ANGEWANDTE CHEMIE", 82, 1970, pages 53-63; J. Oil Col. Chem. Assoc. 1970, 53, pages 363-379; Advan. Urethane Sci. Technol. 4, 1976, pages 112-131).
In some of these literature references, it is also indicated that polyisocyanates may be incorporated in the aqueous polyurethane dispersions; German Offenlegungsschrift No. 1,495,847, for example, recommends inter alia the addition of polyisocyanates as a cross-linking agent before the dispersions are worked-up into shaped products. U.S. Pat. No. 3,756,992 also contains a reference to the fact that free polyisocyanates having molecular weights of from 110 to approximately 4000 may be added to the dispersions described therein. This addition of polyisocyanates is obviously also intended to bring about cross-linking of the dispersed polyurethanes.